Authors: Duran, A; Cardona Lopez, D

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Duran, A & Cardona Lopez, D 2020, 'Toe rock mass strength in footwall failures', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 749-762,

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This paper presents results of check and back-analysis of highwall slopes and footwall instabilities in coal bearing sedimentary strata. The primary aim being to address the issue of rock mass strength at the footwall toe. An issue highlighted previously by one of the authors is the often incomplete knowledge in a footwall scenario to enable analytically based design. Experience gained at coal mining operations in moderate to steeply dipping strata coupled with studies to resolve the unknowns has resulted in an understanding of the critical mechanism, shear strength on bedding defects, groundwater conditions and an approach to assessing the rock mass strength at the toe. This paper focuses on the latter aspect of toe rock mass strength. The results question the applicability of studies suggesting the use of the disturbance factor associated with unloading of slopes to sedimentary strata. The methodology proposed by the authors for the footwall toe rock mass strength to account for anisotropy is to accept a disturbance factor of zero and downrate the Geological Strength Index (GSI) value. Furthermore, the authors caution against the use of the GSI fissile molasse chart for coal bearing strata unless there is significant tectonic deformation of the rock mass evident at the small-scale.

Keywords: footwall, steep dips, coal mining, rock mass strengths, disturbance factor, back-analysis

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